(1) Field of the Invention
The present invention relates to an insert molding device. More particularly, it relates to a device for coating the surface of a workpiece with plastic by means of insert molding.
(2) Description of the Prior Art
Recently, a jaw of a shifter fork, one of the parts of the transmission of an automobile, has been coated with plastic by means of insert molding in order to increase resistance to wear.
The problem of coating the jaw of a shifter fork with plastic in such a manner, results from the clearance between the mold and the workpiece inserted therein.
Because a number of inserted workpieces differ in shape and dimension, it is necessary to increase the clearance between the mold and the inserted workpiece. If the clearance is not increased, some of the workpieces cannot be inserted into the mold.
This increase in clearance naturally causes casting fins to form due to excess plastic entering the clearance after the plastic is injected into the mold.
The length of the casting fins varies depending on the shape and the dimensions of the inserted workpiece and the injection conditions, for example, the composition of the plastic, the temperature of the plastic, and the injection pressure of the plastic.
As a countermeasure to the formation of casting fins, for example, there is known a method in which a pair of molds is provided with sliding keys so that the clearance between the molds and the inserted workpiece is minimized.
According to this method, the workpiece inserted between the pair of molds is pressed down by a clamp pin, and sliding keys are disposed on each side of the inserted workpiece, the sliding keys sliding toward each other due to the descending of cams, mounted on the upper mold, when the molds are closed. Consequently, the clearance disappears. However, in this method, if any of the edges of the inserted workpieces are rounded, a clearance occurs in spite of the use of sliding keys.
A steel plate product, a forged product, and a cast product all will typically have rounded edges, and according to this method, the clearance due to the rounded corners cannot be eliminated.
Therefore, when molding is carried out under such conditions, casting fins are formed on the edges of the inserted work.
These casting fins are thin and do not adhere to the inserted workpiece and therefore must be cut off.
However, because it is difficult to cut them off by means of an automatic machine, they must be cut off manually, thereby decreasing productivity and increasing the cost of the product.